Pentafluoroethane (CF3CHF2) is used, as examples, as a low temperature refrigerant or an etching gas and is also used as a starting material for the production of hexafluoroethane (CF3CF3).
For the production of pentafluoroethane, the following methods have heretofore been known, as examples,
(1) a method of fluorinating tetrachloroethylene (CCl2═CCl2) or a fluoride thereof with hydrogen fluoride (see, Japanese Unexamined Patent Publication No. 8-268932 (JP-A-8-268932)),
(2) a method of reducing and hydrogenating chloropentafluoroethane (CF3CClF2) (see, Japanese Patent No. 2540409), and
(3) a method of reacting a fluorine gas with a halogen-containing ethylene (see, Japanese Unexamined Patent Publication No. 1-38034 (JP-A-1-38034)).
The pentafluoroethanes produced by these methods contain various impurities such as hydrochlorocarbons (HCC), chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC) and hydrofluorocarbons (HFC).
In order to obtain high-purity pentafluoroethane, these impurities must be removed as much as possible. Among these impurities, various purification methods have been proposed to remove chlorofluorocarbons not only for attaining high purity but also for preventing the depletion of the ozone layer. Chloropentafluoroethane is a compound having a boiling point close to that of pentafluoroethane and difficult to separate by ordinary distillation, however, for example, the following purification methods may be used therefor:
(1) a method by extractive distillation (see, Japanese International Application Domestic Publication No. 9-508626 (JP-A-9-508626)),
(2) a method of reducing and hydrogenating chloropentafluoroethane (see, Japanese Unexamined Patent Publication No. 8-301801 (JP-A-8-301801)),
(3) a method of removing chloropentafluoroethane after fluorinating it with hydrogen fluoride (HF) (see, Japanese Unexamined Patent Publication No. 2001-48816 (JP-A-2001-48816)), and
(4) a method of removing chloropentafluoroethane after adsorbing it by using an adsorbent (see, Japanese Unexamined Patent Publication No. 6-92879 (JP-A-6-92879)).
Chloromethane (CH3Cl), which is one of hydrochlorocarbons, forms an azeotropic mixture or azeotrope-like mixture with pentafluoroethane and this compound is very difficult to separate from the pentafluoroethane. Furthermore, difluoromethane (CH2F2) and 1,1,1-trifluoroethane (CF3CH3), which are hydrofluorocarbons, each form an azeotropic mixture or azeotrope-like mixture with pentafluoroethane and these compounds are very difficult to separate from the pentafluoroethane.
As for the method of purifying and thereby removing impurities comprising these difficult-to-separate hydrochlorocarbons or hydrofluorocarbons, for example, a purification method by extractive distillation and a purification method of removing these impurities by the adsorption using an activated carbon are known. However, the purification method by extractive distillation has a problem that a plurality of expensive facilities such as distillation tower are necessary and the equipment cost rises highly. Also, the purification method by the adsorption using an activated carbon cannot provide a sufficient effect.